In a Code-Division Multiple Access (CDMA) cellular communication system, one of the main impairments to system performance is the presence of multipath propagation. Multipath can cause deep fading of the signal envelope, and, intersymbol interference (ISI) in the case for which the multipath delay spread is significant compared with the transmission interval (for example, the chip interval in a CDMA communication system). Additionally, in the downlink, multipath propagation destroys the orthogonality between the desired signal to the mobile in question and the signals to other users on the base station serving this mobile. The resulting interference is referred to as Multiple Access Interference (MAI).
One approach to mitigating the effects of ISI and MAI is to use an equaliser at the mobile receiver. This can be used to some extent to “undo” the effects of multipath propagation on the transmitted signal, and hence go some way to restoring the orthogonality of same-cell signals prior to despreading.
In recent years there has been increasing interest in the use of “software modem” techniques in the field of cellular mobile telecommunications. The principle behind the software modem approach is to move a significant portion of the signal processing functionality from dedicated hardware, as it would have been implemented previously, into software running on a processor of the mobile terminal or other transmitter or receiver. One such function is the equalisation processing.
However, a price in terms of processing complexity must be paid for the use of a software equaliser, both in the calculation of the equaliser coefficients and in the actual equalisation of the received signal, e.g. filtering in the case of linear equalisation. It is therefore desirable to keep the equaliser design constrained to some extent, to avoid excess complexity and its attendant problems with processing capacity and power consumption.
It is an aim of the present invention to find an efficient way of constraining the processing cost of equalisation without unduly reducing performance.